Robert met Jeff Marx at a theatre workshop, and the two of them went on to write the award-winning Broadway musical Avenue Q for which they both won Tony Awards in 2004. He and Jeff have also composed music for the TV series Rick & Steve the Happiest Gay Couple in All the World and Johnny and the Sprites.

In 2008 Robert received a Daytime Emmy Award for Outstanding Achievement in Music Direction and Composition for the TV series The Wonder Pets. In 2011 the musical The Book of Mormon premiered on Broadway, winning Robert two more Tonys and his first Grammy for Best Musical Theater Album.

In 2013 the Disney film Frozen premiered, featuring songs written by Robert and his wife Kristen Anderson-Lopez. At the 2014 Academy Awards the Lopezs received the Oscar for Best Original Song for "Let It Go", making Robert Lopez the youngest person to ever EGOT — to win an Emmy, Grammy, Oscar, and Tony award — and the first to EGOT in only one decade.